1. Field of the Invention
The present invention relates to a heater control device, an image forming apparatus, and a heater controlling method.
2. Description of the Related Art
Some electrophotographic image forming apparatuses use a fixing unit having a halogen heater. In such a halogen heater, a problem of break of a tungsten filament is likely to occur, especially in standby mode. Standby mode is an operation mode in which decreased power is supplied to units of the apparatus in comparison to normal mode in which a normal level of power is supplied. In the standby mode, at least the power supplied to the halogen heater is reduced.
From the viewpoint of reducing electricity consumption, the standby mode is used to control a halogen heater so that the halogen heater is kept ON with the lowest power that is needed to maintain a target temperature. However, with such a control, color temperature of the tungsten filament in the halogen heater does not reach the color temperature to realize a stable condition that is called a halogen cycle, in which halogen gas density in the halogen heater be kept uniform. Therefore, the halogen gas density in the halogen heater has been excessive to cause a phenomenon called a chemical attack, which has deteriorated a tungsten filament to cause to break.
In contrast, a tungsten filament in a halogen heater of a printing apparatus rarely breaks. This is because a fixing unit of the printing apparatus is often controlled in such a way that a sufficiently large amount of electricity is supplied to the fixing unit to fix toner onto a printing sheet in a printing process.
To prevent the tungsten filament from making a break in a halogen heater, Japanese Patent Application Laid-open No. H08-202200, for example, suggests a technique for switching a power supply to the halogen heater between ON and OFF such that the halogen heater is turned off after a rising in the color temperature of the tungsten filament is detected.
In Japanese Patent Application Laid-open No. H08-202200, a control is performed so that the halogen heater is turned ON to allow the halogen cycle to take place even during the standby mode and is turned off when the color temperature reaches the temperature at which the halogen cycle takes place. Therefore, it has been known that an excessive power is consumed or flickering gets worsened even in the standby mode.